Magnesium Oxide: Uses, Benefits, Dosage & Safety
Last updated: August 2026 · Reviewed by the FactoWiki Editorial Team for clarity and source accuracy
Quick summary
Magnesium oxide is the cheapest form, has the highest elemental magnesium by weight, and is the worst absorbed — studies put bioavailability around 4%. It is genuinely useful as a laxative and a poor choice for correcting a deficiency.
What is Magnesium Oxide?
Magnesium bound to oxygen. It carries roughly 60% elemental magnesium by weight, the highest of any common form, which is why labels can show impressive numbers. Very little of it is actually absorbed.
What Magnesium Oxide is commonly used for
Constipation, indigestion and heartburn (as an antacid), and — less appropriately — as the magnesium in cheap multivitamins and supplement blends.
How Magnesium Oxide works
The small absorbed fraction acts as magnesium does. The larger unabsorbed fraction stays in the intestine and draws water in osmotically, producing the laxative effect. Poor absorption is the mechanism of its main use and the reason it fails at its advertised one.
What the evidence says
Each claim below is tied to the specific study behind it, rather than to a general reading list.
- Magnesium oxide showed markedly lower bioavailability than organic magnesium salts. Firoz & Graber, Magnesium Research, 2001 — comparative bioavailability study. (PubMed)
- Magnesium oxide is effective for chronic constipation. Mori et al., Journal of Neurogastroenterology and Motility, 2019 — randomised controlled trial. (PubMed)
- Bioavailability of magnesium oxide is approximately 4%. Comparative absorption literature on magnesium salts. (PubMed)
Typical dosage used in studies
For constipation, 400–1,000 mg of the compound. For repletion it is a poor choice regardless of dose — use glycinate, citrate, malate or taurate.
Side effects and safety
The laxative effect is the main issue and is dose-dependent. Long-term high-dose use risks electrolyte disturbance. Accumulation is a genuine danger in reduced kidney function.
Medication interactions and who should avoid Magnesium Oxide
Medication & safety check
Reduces absorption of levothyroxine, bisphosphonates, tetracyclines, quinolones and some HIV medications. As an antacid it raises gastric pH, affecting drugs that need acid for absorption. Separate by at least two hours.
General information, not personal medical advice. If you take any medication, confirm it is safe to combine with Magnesium Oxide with your doctor or pharmacist first.
Sources & further reading
The claim-level citations above are drawn from these sources:
- Firoz & Graber, Magnesium Research, 2001 — comparative bioavailability study — find on PubMed
- Mori et al., Journal of Neurogastroenterology and Motility, 2019 — randomised controlled trial — find on PubMed
- Comparative absorption literature on magnesium salts — find on PubMed
- NIH Office of Dietary Supplements
- NIH National Center for Complementary and Integrative Health
Frequently asked questions
Why is it in so many supplements?
It is the cheapest source of elemental magnesium and lets a label show a large number. That is a cost decision, not a formulation one.
Is it useless?
Not at all — it works well as a laxative and antacid. It is simply the wrong tool for correcting a magnesium shortfall.
How do I spot it?
Read the form on the panel, not just the milligrams. If it says oxide and you are taking it for sleep, cramps or repletion, switch.
Part of a wider guide
Magnesium Oxide is one form of Magnesium. This page covers what is specific to this form — absorption, tolerability, dosing and who it suits. For the underlying nutrient itself, its overall evidence base and general safety, read the Magnesium guide.
Where you'll find Magnesium Oxide
Magnesium Oxide appears in Minerals & Digestive Support formulas. Browse the supplement guides to see the products we review, each with a full breakdown of formula, pricing and safety.
Related ingredients to explore
Ingredients often studied or formulated alongside Magnesium Oxide.
General information, not medical advice.